This invention relates to a vehicle drive system.
JP-A-H11-78620 published by the Japanese Patent Office in 1999 discloses a vehicle drive system. This system is used in a vehicle mounting a continuously variable transmission. A target engine torque is calculated by dividing a target drive force by the actual speed ratio. In this manner, an optimal target engine torque is produced which constantly corresponds to the speed ratio without the need for a complicated control routine. Furthermore the step change of the drive force before and after the speed change is suppressed to a minimum.
Acceleration requirements (target drive force) when the vehicle is reversing are lower than acceleration requirements when the vehicle is moving forward. Thus, if the drive force control is also performed when the vehicle is reversing, performance characteristics, which are different from the characteristics for forward running, are required during reverse running.
In this context, it has been proposed to prepare respective target drive force maps for forward and reverse vehicle operation as a method of differentiating from performance characteristics used during forward vehicle performance. However control routines are complicated by the need to switch between the maps used for forward and reverse vehicle operation. Furthermore this arrangement also has a disadvantage that the capacity of a ROM must be increased in order to store the designated reverse operation target drive force map.
It is therefore an object of this invention to realize drive force control corresponding to low acceleration requirements of a driver during reverse vehicle operation by using a simple structure. It is a further object of this invention to suppress the generation of shift shocks when switching between forward and reverse vehicle operation.
In order to achieve above object, this invention provides a vehicle drive system comprising an engine, an automatic transmission connected to the engine, a sensor which detects a running condition of the vehicle, a sensor which detects an operational condition of the transmission, and a controller. The controller functions to calculate a target drive force based on the running condition of the vehicle, calculate a first value based on the state of the transmission, set the first value as a speed ratio for calculating a target engine torque during forward vehicle operation, set a second value calculated by correcting the first value to a larger value as the speed ratio for calculating the target engine torque during reverse vehicle running, calculate the target engine torque by dividing the target drive force by the speed ratio for calculating the target engine torque and control the engine so that the engine torque coincides with the target engine torque.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.
FIG. 1 is a schematic diagram of a vehicle drive force system according to this invention.
FIG. 2 is a schematic diagram of a power train for the vehicle.
FIG. 3 is a block diagram of drive force control.
FIG. 4 is a table for calculating the torque converter torque ratio.
FIG. 5 is a map for calculating the target throttle opening.
FIG. 6 is a block diagram showing the calculation process of a speed ratio for calculation of the target engine torque.
FIG. 7 is a flowchart showing a limiting process on the varying speed of the speed ratio for calculation of the target engine torque performed during switching between forward and reverse vehicle operation.
FIG. 8 is a time chart showing variation in the speed ratio for calculation of the target engine torque under the speed limitation when the select lever is operated from the D range (first gear) to the R range.
FIG. 9 is a time chart showing respective variation in the drive force, the speed ratio and the target engine torque when the select lever is operated from the D range to the R range.